This invention relates to multigrain compacts of diamond or cubic boron nitride particles bonded by a sintered matrix material.
A compact is a mass of abrasive particles that is bonded together in a self bonded relationship as described in U.S. Pat. Nos. 3,852,078 and 3,876,751; by means of a bonding medium, as described in U.S. Pat. Nos. 3,136,615, 3,233,988, 3,743,489, 3,767,371 and 3,918,931 or by means of some combination thereof. The abrasive particles used can have single grain (monocrystalline) or multigrain (polycrystalline) crystal structure. Monocrystalline particles are formed with the aid of a catalyst while polycrystalline particles are obtained by direct conversion of a compound having an alternate crystal structure such as hexagonal boron nitride and graphite. Representative of these compacts are U.S. Pat. Nos. 2,367,404, 3,650,714, 3,957,461, 3,929,432, 3,984,214, 4,224,380, 4,288,248 and 4,738,689.
Compacts of cubic boron nitride can be formed simultaneously with the conversion of hexagonal boron nitride to cubic boron nitride. This can be done with or without the aid of a catalyst metal or alloy. One example of a method which does not require a catalyst is described in copending U.S. patent application Ser. No. 07/365,883, filed Jun. 14, 1989, and assigned to the same assignee as the present invention. In the manufacture of prior art compacts, high temperatures and pressures greater than 50 kbar are required to obtain interparticle bonding and/or conversion of HBN to CBN.
From the foregoing, it can be seen that it would be advantageous to form multigrain compacts at lower temperatures and pressures, particularly since high pressure/high temperature apparatus represent significant capital equipment expense and their use requires a significant expenditure of energy.
It is well known in the art that metal coatings on abrasive particles such as diamond and cubic boron nitride improve the retention of such particles in the matrices of abrasive tools such as grinding wheels and similar agglomerated aggregates. For example, it is preferable to use a nickel coating on diamond and cubic boron nitride particles to improve their retention within a resin matrix of a grinding wheel. In copending U.S. patent application Ser. No. 07/365,883, a coating to prevent oxidation is used on the HBN particles which is said to help bond the particulates. However, until now, coated granules have not been used to provide abrasive compacts under relatively mild process conditions.